1. Field of the Invention
The present invention relates to a liquid crystal display panel for forming an optical image by changing the diffraction of light, its method of manufacture, and a projection display apparatus for magnifying the optical image displayed on the liquid crystal display panel and projecting the image onto a screen.
2. Description of the Prior Art
A large screen display is recently attracting attention, such for use as a home theater and for use in making a presentation. Many projection apparatus using light valve have been proposed. Lately, liquid crystal projection apparatus for displaying an image on a wide screen using a small liquid crystal display panel and a projection lens or the like, have been developed.
The liquid crystal display panel is designed to create a display image mainly by varying its optical characteristic electrically, and is classified based on many kinds of operating principles. A twisted nematic (TN) liquid crystal display panel used in existing liquid crystal projection apparatus makes use of an electric field to change the polarization of the liquid crystal. The TN liquid crystal display panel, however, requires a polarizer for modulating the light, and hence the efficiency is poor.
Known methods of controlling the light without using a polarizer include methods based on using the scattering or diffraction of light. Liquid crystal display panels for forming an optical image by varying the state in which light is scattered include phase change (PC), dynamic scattering mode (DSM), and polymer dispersion liquid crystal panels. In particular, the polymer dispersion liquid crystal panel has been under intensive consideration recently. Such a panel is disclosed, for example, in U.S. Pat. No. 4,435,047. However, in a liquid crystal projection apparatus using this polymer dispersion liquid crystal panel there is a compromise between the image brightness and contrast (Tomita, Proc. of SID, p. 579, 1992), and such an apparatus capable of forming an image having both satisfactory brightness and contrast has not yet been realized.
On the other hand, a liquid crystal display panel for forming an optical image by changing the state under which light is diffracted is disclosed in U.S. Pat. No. 4,729,640. The basic structure and operation of this type of liquid crystal display panel are shown in FIGS. 13(a) and 13(b). Transparent electrodes 135 and 136 are formed on glass substrates 131 and 132, and an irregular sectional layer 134 is formed on a surface of at least one of the glass substrates confronting a liquid crystal layer 133. The irregular sectional layer 134 has a periodic configuration, and the refractive index of the irregular sectional layer 134 is nearly equal to the ordinary refractive index no of a liquid crystal 137. FIG. 13(a) shows a case in which an electric field is not applied to the liquid crystal layer 133, and the liquid crystal 137 is oriented in a homogeneous state with its molecular long axis parallel to the longitudinal direction of the stripes of the irregular sectional layer 134. A ray of light 138 entering this liquid crystal panel is transmitted with a refractive index of n.sub.o in the convex portion of the irregular sectional layer 134. However, a polarized component 138a of the light is transmitted in the concave portion of irregular sectional layer, i.e. in the liquid crystal, with an extraordinary refractive index of n.sub.c. Thus the irregular sectional layer acts as a phase grating, and the ray of light 138 is modulated. On the other hand, FIG. 13(b) shows a case in which a sufficiently large electric field is applied to the liquid crystal layer 133 so that the liquid crystal layer 137, having a positive dielectric anisotropy, is oriented in a direction normal to the glass substrates 131, 132. Accordingly, the light is transmitted in the concave portion, i.e. in the liquid crystal 137, with an ordinary refractive index n.sub.o which is the same as the refractive index of the convex portion. Accordingly, the incident ray of light 138 is not diffracted but propagates straight through the panel.
An example of a projection display apparatus using the diffraction type of liquid crystal display panel of FIG. 13 is shown in FIG. 14. The light emitted from a lamp 141 is converted by a concave mirror 142, passes through a polarizer 145, and enters a liquid crystal display panel 143. Natural light emitted from the lamp 141 is half-absorbed by the polarizer 145, and the polarized light enters the liquid crystal display panel 143. The light entering the liquid crystal panel 143, if not modulated, is completely led into a projection lens 144. A matrix pixel electrode and a grating are provided on one side of the liquid crystal layer 133 of the liquid crystal panel 143. An optical image can be formed on the liquid crystal panel 143 by changing the state of diffraction of the light with video signals. The light transmitted from a pixel to which a sufficient voltage is applied completely enters the projection lens 144 and reaches a screen 148, and a bright spot is displayed at a corresponding position on the screen 148. Diffracted light is emitted from the pixels across which no voltage is applied. The diffracted light is transmitted from the projection lens 144 and does not reach the screen 148, and a dark spot is displayed at a corresponding position on the screen 148.
Problems of the conventional diffraction type of liquid crystal panel are discussed below. While the liquid crystal display panel is in the "diffraction state" the polarized component 138a of the light 138, oscillating in a direction perpendicular to the sheet of FIG. 13, is transmitted through the liquid crystal with the extraordinary refractive index n.sub.e, and the ray of light is diffracted and modulated. However, a polarized component 138b of the light 138, oscillating in a direction parallel to the sheet of paper, passes through the liquid crystal with an ordinary refractive index n.sub.o. Hence, the ray of light is not modulated. That is, only 50% of the incident ray of light is diffracted and modulated, while the remaining 50% of the ray of light directly passes through the panel. Therefore, in the projection type display apparatus shown in FIG. 14, the polarized 145 is used to transmit only polarized light capable of being diffracted and modulated by the liquid crystal panel 143. Hence, the efficiency of light utilization is 50%.
To solve this problem, a diffraction type of liquid crystal display panel has been proposed in U.S. Pat. No. 4,251,137. In this panel, gratings are formed on upper and lower substrates, respectively, and are oriented orthogonally. U.S. Pat. No. 4,856,869 similarly discloses an apparatus in which two diffraction liquid crystal display panels are arranged so that the gratings of the respective panels are orthogonal.
However, each grating in these panels must have an approximate height of several microns, and a period of about several microns to about 20 microns. It is extremely difficult to form the gratings uniformly over the entire display area.
Further, if the grating is formed of a substance having a dielectric constant different from that of the liquid crystal, when a sufficient electric field is applied between upper and lower electrodes, the direction of electric lines of force is inclined toward that element having the lower dielectric constant. Accordingly, the liquid crystal aligns in the direction of the electric line of force, whereupon the refractive index of the liquid crystal does not match the grating.
Moreover, if the refractive indices of the liquid crystal and the grating are matched for a certain wavelength of light, light of another wavelength may be transmitted through the liquid crystal and grating with different refractive indices. In this case, the light may be diffracted.
Similarly, if the refractive indices of the liquid crystal and grating are matched at a certain temperature, light may be transmitted with different refractive indices due to temperature changes, thereby causing diffraction.